The present invention relates to physiological monitoring instruments and, in particular, monitors and sensors that include mechanisms for indicating a quality of detected signals and accuracy or confidence level of physiological measurements estimated from the signals.
Typically, for physiological monitoring instruments that include a monitor and a patient sensor, the monitor is unable to accurately determine a quality of a signal obtained from the sensor. The invention will be explained by reference to a preferred embodiment concerning pulse oximeter monitors and pulse oximetry sensors, but it should be realized the invention is applicable to any generalized patient monitor and associated patient sensor. The invention provides a way of more accurately determining a quality of a signal detected by a sensor; a way of determining a relative accuracy of a physiological characteristic derived or calculated from the signal; and a way of delineating a transition boundary between a normal signal for the sensor being used in its normal application, and a signal considered to be abnormal for the sensor being used, to allow a monitor to determine if the sensor is being misapplied.
Pulse oximetry is typically used to measure various blood flow characteristics including, but not limited to, the blood oxygen saturation of hemoglobin in arterial blood and the heartbeat of a patient. Measurement of these characteristics has been accomplished by the use of a non-invasive sensor that passes light through a portion of a patient""s blood perfused tissue and photo-electrically senses the absorption and scattering of light in such tissue. The amount of light absorbed and scattered is then used to estimate the amount of blood constituent in the tissue using various algorithms known in the art. The xe2x80x9cpulsexe2x80x9d in pulse oximetry comes from the time varying amount of arterial blood in the tissue during a cardiac cycle. The signal processed from the sensed optical signal is a familiar plethysmographic waveform due to the cycling light attenuation.
The light passed through the tissue is typically selected to be of two or more wavelengths that are absorbed by the blood in an amount related to the amount of blood constituent present in the blood. The amount of transmitted light that passes through the tissue varies in accordance with the changing amount of blood constituent in the tissue and the related light absorption.
To estimate arterial blood oxygen saturation of a patient, conventional two-wavelength pulse oximeters emit light from two light emitting diodes (LEDs) into a pulsatile tissue bed and collect the transmitted light with a photodiode (or photo-detector) positioned on an opposite surface (i.e., for transmission pulse oximetry) or an adjacent surface (i.e., for reflectance pulse oximetry). The LEDs and photo-detector are typically housed in a reusable or disposable oximeter sensor that couples to a pulse oximeter electronics and display unit. One of the two LEDs"" primary wavelength is selected at a point in the electromagnetic spectrum where the absorption of oxyhemoglobin (HbO2) differs from the absorption of reduced hemoglobin (Hb). The second of the two LEDs"" wavelength is selected at a different point in the spectrum where the absorption of Hb and HbO2 differs from those at the first wavelength. Commercial pulse oximeters typically utilize one wavelength in the near red part of the visible spectrum near 660 nanometers (nm) and one in the near infrared (IR) part of the spectrum in the range of 880-940 nm.
Oxygen saturation can be estimated using various techniques. In one common technique, first and second photo-current signals generated by the photo-detector from red and infrared light are conditioned and processed to determine AC and DC signal components and a modulation ratio of the red to infrared signals. This modulation ratio has been observed to correlate well to arterial oxygen saturation. Pulse oximeters and sensors are empirically calibrated by measuring the modulation ratio over a range of in vivo measured arterial oxygen saturations (SaO2) on a set of patients, healthy volunteers, or animals. The observed correlation is used in an inverse manner to estimate blood oxygen saturation (SpO2) based on the measured value of modulation ratios. The estimation of oxygen saturation using modulation ratio is described in U.S. Pat. No. 5,853,364, entitled xe2x80x9cMETHOD AND APPARATUS FOR ESTIMATING PHYSIOLOGICAL PARAMETERS USING MODEL-BASED ADAPTIVE FILTERINGxe2x80x9d, issued Dec. 29, 1998, and U.S. Pat. No. 4,911,167, entitled xe2x80x9cMETHOD AND APPARATUS FOR DETECTING OPTICAL PULSESxe2x80x9d, issued Mar. 27, 1990. The relationship between oxygen saturation and modulation ratio is further described in U.S. Pat. No. 5,645,059, entitled xe2x80x9cMEDICAL SENSOR WITH MODULATED ENCODING SCHEME,xe2x80x9d issued Jul. 8, 1997. All three patents are assigned to the assignee of the present invention and incorporated herein by reference.
The accuracy of the estimates of the blood flow characteristics depends on a number of factors. For example, the light absorption characteristics typically vary from patient to patient depending on their physiology. Moreover, the absorption characteristics vary depending on the location (e.g., the foot, finger, ear, and so on) where the sensor is applied. Further, the light absorption characteristics vary depending on the design or model of the sensor. Also, the light absorption characteristics of any single sensor design vary from sensor to sensor (e.g., due to different characteristics of the light sources or photo-detector, or both). The clinician applying the sensor correctly or incorrectly may also have a large impact in the results, for example, by loosely or firmly applying the sensor or by applying the sensor to a body part which is inappropriate for the particular sensor design being used.
Some oximeters xe2x80x9cqualifyxe2x80x9d measurements before displaying them on the monitor. One conventional technique processes (i.e., filters) the measured plethysmographic waveform and performs tests to detect and reject measurements perceived corrupted and inaccurate. Since oximeters are typically designed to be used with a wide variety of sensors having widely differing performance characteristics, the monitor signal xe2x80x9cqualificationxe2x80x9d algorithms are necessarily crude, and often result in only superficial indications of signal quality, signal reliability, and ultimately a confidence level in a patient physiological characteristic estimated or calculated from the signal. In many instances, the monitor simply discards data associated with low quality signals, but otherwise gives no indication to a healthcare giver as to whether any physiological characteristic displayed on a monitor is highly reliable or not. Hence, the signal quality measurements obtained from such crude algorithms are relatively poor and convey little useful information to a caregiver.
Accordingly, it is an object of the present invention to provide a patient monitor and sensor which includes means for accurately detecting a quality of a signal detected by the sensor.
Another object of the invention is to provide a monitor and sensor which includes means for accurately determining a quality of a physical characteristic estimated from a signal obtained by a sensor.
A further object of the invention is to provide a monitor and sensor which includes means for detecting a transition between a signal regime considered normal for the sensor in its usual application, and a signal regime considered to be abnormal.
These and others objects of the invention are achieved by the use of a set of one or more signal specification boundaries. Each boundary defines a region of a signal quality diagram and corresponds to a different level of quality in the detected signals and accuracy or confidence level of physiological characteristic estimated from the detected signals. Boundaries can also be defined for and associated with different sensor types and monitor types. The boundaries are typically stored in a memory and accessed when required.
An embodiment of the invention provides a sensor for sensing at least one physiological characteristic of a patient. The sensor is connectable to a monitor that estimates a physiological condition from signals detected by the sensor. The sensor includes a detector for detecting the signals from the patient which are indicative of the physiological characteristic. The sensor is associated with a memory configured to store data that defines at least one sensor signal specification boundary for the detected signals. The boundary is indicative of a quality of the signals and an accuracy of the physiological characteristic estimated from the signals by the monitor. The sensor further includes means for providing access to the memory to allow transmission of the data that defines the at least one sensor boundary to the monitor.
In an embodiment, the boundary is indicative of a transition between a signal regime considered normal for the sensor in its usual application, and a signal regime considered to be abnormal. The normal regime can be one in which the sensor is likely to be properly applied to the patient and the abnormal regime can be one in which the sensor may have partially or entirely come off the patient.
Another embodiment of the invention provides a monitor for providing an indication of an accuracy of an estimated physiological condition of a patient. The monitor is connectable to a sensor that detects signals indicative of at least one physiological characteristic of the patient. The monitor includes at least one receiving circuit and at least one processing circuit. The receiving circuit is configured to receive the signals indicative of the at least one physiological characteristic and data defining at least one sensor signal specification boundary for the detected signals. The processing circuit is configured to estimate the physiological condition of the patient based on the received signals, compare the received signals against the at least one sensor boundary, and generate the indication of the accuracy of the estimated physiological condition. The monitor further includes means for providing the indication of the accuracy of the estimated physiological condition to a user of the monitor.
Yet another embodiment of the invention provides a pulse oximetry system that includes the sensor described above and a pulse oximetry monitor. The monitor has means to determine whether the signals are within a normal regime or an abnormal regime. The system further includes means for informing a user of the system as to whether the signal is normal or abnormal.